1. Field of the Invention
The invention relates generally to a retorting process for obtaining oil from hydrocarbon-containing solids, such as oil shale. The invention relates specifically to that part of a retorting process in which fines are removed from the liquid oil produced in the process.
2. Description of the Prior Art
Oil shale is a fine-grained sedimentary rock that contains an organic material called kerogen. Upon strong heating of the raw shale, the kerogen will decompose or pyrolyze to yield liquid oil, gases, and residual carbon that remains on the shale. The process of pyrolyzing the kerogen in the raw shale is known as "retorting."
Retorting of raw oil shale is basically a simple operation which involves heating the raw oil shale to an appropriate temperature and removing the vapors evolved from the kerogen pyrolysis. The overhead vapors typically contain varying amounts of entrained particles (commonly called fines or dust) depending upon the particulate nature of the shale feed and the kind of retorting process involved. It is usually necessary to remove the fines from the overhead vapors to avoid contaminating the liquid oil product and rendering it unsuitable as a feed to conventional refining operations. In many retort processes the fines in the overhead vapors are of such size and concentration that they may be removed effectively in one or more solid-vapor separation steps. Devices such as hot cyclone separators and electrostatic precipitators have been used or suggested for this purpose.
U.S. Pat. No. 4,133,739 exemplifies a fluidized bed retorting process in which fines are separated from the overhead vapors and are recycled to the retort vessel. Retorting occurs in an upper portion of the retort vessel and combustion of the char on the retorted shale to provide the heat of retorting occurs in the lower portion of the retort vessel. The fines entrained in the overhead stream are separated and returned to the combustion portion of the vessel where the char thereon is burned to supply a portion of the heat for retorting. The patent teaches a condensation step that follows the solid-vapor separation to recover the liquid oil from the dedusted gas vapor phase. There is no mention of further treatment of the condensate to remove any solids that might be present therein.
U.S. Pat. No. 4,199,432 describes a staged turbulent bed retort process that may produce substantial concentrations of fines in the retort vapors depending upon the particular feed and operating conditions used. Raw hydrocarbon-containing solids are fed together with a hot heat transfer material that supplies the heat for retorting down through a retort vessel while a nonoxidizing stripping/fluidizing gas is passed countercurrently and upwardly through the vessel. The retorted solids and heat transfer material are taken from the bottom of the vessel and passed to a combustor where the char on the retorted solids is combusted to heat the heat transfer material. The retort vapors together with the stripping gas are taken overhead from the retort vessel and subjected to a conventional solid-vapor separation to remove entrained fines. The fines are returned to the bottom of the combustor. The dedusted vapor stream is then condensed and the condensate is distilled into product.
The "Lurgi-Ruhrgas" retort process involves a feed (particle size usually below one cm nominal diameter) and retorting procedures that give rise to substantial amounts of fines in the overhead vapors. The Lurgi-Ruhrgas process is similar to the process of U.S. Pat. No. 4,199,432 described above in that retorted solids are passed to a combustor where the char is combusted to generate heat and raise the temperature of a heat transfer material. The vapor product from the retort is subjected to a solids separation and then to a multi-stage condensation. Literature relating to this process suggests either recycling the heavy oil from the first condensation stage to the retort to dedust and crack it, recycling the heavy oil to the combustion zone as a combustion fuel, or centrifuging the heavy oil after treatment with a solvent, drying the centrifuge residue with steam, and disposing of the dried sediment.
U.S. Pat. No. 4,226,699 describes an oil shale retorting process in which a sludge recovered from the retort vapors is combusted to heat flue gases that are used to preheat the raw oil shale. More specifically, this process involves preheating the oil shale in a series of lift pipes by contact with hot flue gases. The preheated shale is retorted and the retort vapors are taken overhead and condensed. The condensate is subjected to a liquid-solids separation to form the combustable sludge. This sludge is pumped to a combustor positioned between the last and penultimate lift pipes where it is mixed with flue gases from the last lift pipe and combusted to heat those gases.